Individual's suffering from pulmonary edema, i.e., the effusion of serous fluid into the lungs, and certain other respiratory ailments are generally treated by forcing breathable gas, normally oxygen (O2) into the lungs and maintaining the pressure within the lungs at a level, e.g., 1 to 20 centimeters of water above atmospheric. The O2 can be supplied directly to the lungs through an endotracheal tube, one end of which is inserted into the lungs through the individual's mouth, i.e., intubation. The invasive technique of intubation requires considerable skill and can cause serious injury to the patient. Also, the recovery time of intubated patients may be considerable.
Alternatively, a patient may be fitted with a breathing appliance such as a face mask which is equipped with an inlet for receiving oxygen under pressure and an inhalation/exhalation valve for exhausting exhaled air to the atmosphere. The respiratory departments of many hospitals have relatively sophisticated equipment for supplying oxygen at continuous and/or dual level pressure to such appliances. However, such equipment is neither readily portable nor simple to operate and often is not available in emergency rooms.
Portable systems are currently available for use in emergency rooms by nurses and in the field by emergency rescue personnel, e.g., paramedics, for the continuous positive airway pressure (“CPAP”) procedure. However, such portable systems conventionally rely on a spring loaded check valve located in or near the face mask to set the maximum pressure in the mask. The check valve serves to bypass the oxygen stream to the atmosphere during the patient's exhalation phase. The flow rate is normally adjusted to accommodate a patient's peak inhalation flow rate, e.g., 75 to 100 liters per minute (l/m). A patient typically inhales around 10 to 12 l/m with each exhalation phase exceeding the time duration of the inhalation phase by a factor of two or more.
As a result, currently available portable systems for use by emergency rescue personnel consume oxygen at a high rate stemming from the fact that they are continuous flow devices that must cater to high demand and waste O2 during the longer expiration phase of the respiratory cycle. Also, this high flow rate creates unwanted additional expiratory work for the patient.
In a normal respiratory cycle the torso muscles act to expand the lungs and thus draw air into them during the inhalation cycle. Exhalation is accomplished by the muscles relaxing and the elastic recoil of the chest forcing air from the lungs. During positive pressure breathing the muscle action is reversed so that air enters the lungs under pressure and exhalation requires forceful action by the abdominal muscles. Thus, exhalation under conventional CPAP treatment involves a significant amount of exertion for the patient.
The shock to a patient being suddenly confronted with a significant amount of pressure in his or her airway, e.g., 10 to 20 cm H2O during inhalation/exhalation is another disadvantage of the currently available portable CPAP systems.
U.S. Pat. No. 5,148,802 and related U.S. Pat. Nos. 5,433,193 and 5,632,269, while not directed to portable CPAP systems for use by emergency rescue personnel, disclose a sophisticated system (“'802 system”) employing the CPAP treatment for individuals suffering from sleep apnea. The '802 system, which is designed to keep the individual's airway continuously open during sleep, employs a sensitive flow sensor and complicated electronic circuitry to determine when the user is exhaling and lowers the applied pressure during the expiratory phase.
The '802 system is expensive and, as with many complicated electronic devices, would be subject to failure if mishandled.
There is a need for a simple, inexpensive, reliable, portable and rugged apparatus which can be used by emergency rescue personnel whether in the field or in emergency rooms to ventilate a patient's lungs with oxygen under continuous positive airway pressure.